The pentapeptides, Tyr--Gly--Gly--Phe--X[where X=Leu (I) or X=Met (II)] have been isolated and characterized by Hughes et al., Nature 258, 557 (1975). It was shown that they possess analgesic activity only after intracerebral (icv) administration, Belluzzi et al., Nature, 260 625 (1975). Pert et al., Science, 294, 330 (1976 reported that the pentapeptide Tyr--D--Ala--Gly--Phe--Met--NH.sub.2 (III) exhibits potent and prolonged analgesia after icv administration. Bajusz et al., FEBS Leters 76, 91 (1977) by replacing Gly.sup.2 with D--Met and the Met.sup.5 by Pro--NH.sub.2 obtained a very potent antinociceptive pentapeptide Tyr--D--Met--Gly--Phe--Pro--NH.sub.2 (IV) which was 5.5 times more potent than morphine by intravenous administration. Romer et al., Nature, 268, 547 (1977) showed that the substituted tetrapeptide amide (V) possesses potent peripheral analgesic activity and some analgesic activity when given orally at high does (200-300 mg/kg.). Morgan et al., "Peptides, Proc. Fifth Amer. Pept. ##STR2## Symp." ed. Goodman and Meienhofer, p. 111 (1977) reported in vitro and in vivo biological activites of several enkephalin analogs among which was N(Me)Tyr--Gly--Gly--Phe--Met--NH--Propyl (VI). Ling et al., ibid., p. 96 (1977) reported in vitro activities of several analogs of enkephaline with D-amino acids in position 5. Dutta et al., Life Sciences, 21, 559 (1977) and Dutta et al. Acta. Pharm. Sciences, 14, 14 (1977) described several analogs with D--Ser, D--Met, D--Ala, D--Thr, D--Lys (Boc), D--Phe, D--Leu, D--Asp and D--Ser(t-Bu) at position 2 and various substitutions with L-amino acids or amines at position 5. Belluzzi et al., Life Sciences, 23, 99 (1978) described analogs with D--Ala at position 2 and D--Leu or D--Met at position 5.